Importance of matching in expo convertors?!

[Ben Nise]

Being new to this mailing list, I was wondering if PTAT (Proportional to 
absolute temperature) current sources were ever used to compensate for the
temperature dependent gain of exponential voltage to current converters.
(PTAT sources are ubiqutous in IC design to amplifier gains stable over 
temperature and to create voltage references).

I can see how just using a differential pair will create a temperature
stable reference
point, but the gain will still drift over temperature. Once way to prevent
this from 
happening is to put a heater on the differential pair.  Another way would be
to 
use a PTAT current source....

PTAT current sources are easy to build on chip, but I don't have any
experiance
doing this on the board level.  Somthing like the LM134 might work...

My own background is in analog IC design and not analog board design, so I
apologize
if my terminology is unfamiliar, if I make stupid statemants with regard
to board level design technique, or if I mention something that has been
discussed in depth
previously. 

Regards,

Ben Nise
bnise at sonici.com

-----Original Message-----
From: Ben Nise 
Sent: Tuesday, August 31, 1999 7:29 AM
To: 'Rene Schmitz'; Ben Nise
Cc: synth-diy at mailhost.bpa.nl
Subject: RE: Importance of matching in expo convertors?!


Rene --

>>There are other mismatch mechanisms that affect effect BJT's besides Is,
>>namely
>>Beta mismatch and tempco mismatch.  

>Sorry for the stupid question, but what do you mean by tempco mismatch?! 
>That the transistors still need to be on the same die or at least be
>thermally close coupled ?

What I really meant by "tempco" mismatch is that the parametric behavior
of two BJT's (Rb, Beta!!, Is, etc...) may not behave the same way over
temperature
for two devices that are not on the same die.  This would most likely show
up as
a temperature dependant offset -- (even with thermal coupling).

Regards,

Ben Nise
bnise at sonici.com